Most rainfall in volcanic areas of southern Guam enters directly to stream valleys. The island has about 100 streams, which range in length from less than 1 km to more than 5 km. They flow in deeply incised and V-shaped valleys in highland areas, or wide and flat-bottomed valleys in coastal lowlands, wherever the ground surface intersects the water table.

Parts of southern Guam that are covered by limestone contain significant groundwater bodies. Areas where limestone is completely surrounded by volcanic terrain include perched groundwater, which sits within permeable limestone outcrops on top of volcanic rocks of low-permeability. These perched systems are sources of main freshwater springs in southern Guam.